Runoff mostly happens in the form of high runoff discharge events that occur in a very short period. These are related to the steep topography, often little vegetation cover and intense convective rainfall. The peaks of such flash floods have often a 50 to 100 times larger discharge than the preceding baseflow. The magnitude of floods in this river has however been decreased due to interventions in the catchment. In May Be’ati the effect of such interventions has also been monitored.
Physical and biological measures in the catchment
At Gemgema, May Be'ati and on other steep slopes, exclosures have been established; the dense vegetation largely contributes to enhanced infiltration, less flooding and better baseflow. Physical conservation structures such as stone bunds and check dams also intercept runoff. In the May Be'ati exclosure, more than 1000 precise measurements were done in 2003 and 2004, using 15 runoff plots, where the volume of runoff was measured daily. The rock type, slope gradient and slope aspect were the same, the only difference was the land management and vegetation density. Whereas in degraded rangeland, 35% of the rainfall flows directly away to the river, this happens only for 13.4% of the rain in a recent exclosure and 1.7% in an old exclosure. For sake of comparison, the adjacent May Be'ati church forest has a runoff coefficient of only 0.1%.
Check dam construction
The effects of check dams on runoff response have been studied at Addi Qolqwal in the upper catchment of this river, near the road on soils derived from Amba Aradam Sandstone. An increase of hydraulic roughness by check dams and water transmission losses in deposited sediments are responsible for the delay of runoff to reach the lower part of the river channels. The reduction of peak runoff discharge was larger in the torrent with check dams and vegetation than in that without treatment. Reduction of total runoff volume was also larger in the torrent with check dams than in the untreated torrent. The implementation of check dams combined with vegetation reduced peak flow discharge and total runoff volume as large parts of runoff infiltrated in the sediments deposited behind the check dams. As gully check dams are implemented in a large areas of northern Ethiopia, this contributes to groundwater recharge and increased river baseflow.
Check dams and subsurface dams
The most common structure to control gullies in this catchment is the check dam. However, there are several problems with collapsing check dams, even if they are built in a correct way, using well-shaped stones and having spillway and apron. One problem is in the Vertisol areas, such as near May Be’ati village in the upper catchment: sometimes the check dam stands intact and the water creates a new gully around it. The reasons are the cracks occurring in Vertisols: the water that accumulates behind the dam will start flowing through the cracks; it will round the dam and create an underground tunnel that becomes wider and wider until collapse. To control this, simple and cheap subsurface dams were implemented around 2010. Trenches, 2.5 meter deep and 25 meter long, were excavated at both sides of a check dams and a heavy plastic sheet inserted vertically, after which the trench was filled with soil. The plastic sheet prevents water from flowing underground through the cracks. No more bypass occurs, and the water table is raised.
The effectiveness of exclosures could be enhanced by supplementing additional water to the short and erratic rain. The effects of such spate irrigation on species diversity, stocking and ring width growth of trees was evaluated in two exclosures in the catchment of May Be’ati River. The runoff diversion from the gully channel towards the regenerating forest was done with trenches dug at different locations to enhance an even runoff water distribution over the exclosures. The exclosure in May Be’ati was irrigated in 2005 while the Addi Qolqwal exclosure was irrigated from 2012 onwards but monitored from 2014 to 2016. The volume of irrigation water applied is of the same order of magnitude as the direct rainfall on the site. Trees in the irrigated exclosure have greater species diversity and show better growth. Particularly during the peakrainy season when there is excess water in Dogu’a Tembien, that cannot be used for crop irrigation, spate irrigation towards woody vegetation can be an important buffer for peak runoff discharges and if largely applied, it can reduce floods in the downstream areas.
Boulders and pebbles in the river bed
Boulders and pebbles encountered in the river bed can originate from any location higher up in the catchment. In the uppermost stretches of the river, only rock fragments of the upper lithological units will be present in the river bed, whereas more downstream one may find a more comprehensive mix of all lithologies crossed by the river. From upstream to downstream, the following lithological units occur in the catchment.
Trekking routes have been established across and along this river. The tracks are not marked on the ground but can be followed using downloaded.GPX files. Trek routes 13 and 13V cross the river and its’ catchment.
